Upon separation or condensation of only a specific kind of substances out of a fluid where several kinds of substances are mixed together, a separation membrane may be used.
The separation membrane has a structure having countless pores passing therethrough. Though various substances can generally be passed through the pores of the separation membrane, easiness of passing depends on the kind of substances. Therefore, in the case that substances easily passing through the pores of the separation membrane and substances hardly passing through the pores of the separation membrane are mixed together in substances contained in a fluid, by treating the fluid with the separation membrane, substances easily passing through the pores can pass through the separation membrane while substances hardly passing through the pores cannot pass through the separation membrane and remain as they are. Thus, treating a fluid with a separation membrane enables a specific kind of substances contained in the fluid to be separated or condensed.
A silica membrane is a porous membrane having countless pores passing therethrough. Generally, the pores of a silica membrane are characterized by easily passing substances having small molecular diameters (e.g., water and carbon dioxide). Using this characteristic, a silica membrane is used for the case of separating water from a mixed liquid of water and ethanol, the case of separating carbon dioxide from a combustion exhaust gas, or the like (e.g., Patent Documents 1 to 5 and Non-Patent Documents 1 to 4). On the other hand, the pores of a conventional silica membrane are characterized by hardly passing an aromatic compound and alcohol having relatively large molecular diameters.